As shown in FIG. 1, an existing axial fan 900 (the axial fan is a fan in which air comes out along the axial direction of a shaft of the fan) propels the air flow out by rotation of blades 920, where the direction of the air flow is parallel to a rotating axis. The air flow speed becomes higher and higher, approaching top areas of the blades 920; the air flow speed becomes lower and lower, approaching an area of a hub 910 (that is, bottom parts of the blades); and the air flow speed is zero at the hub 910, thereby forming an air flow dead area. As a result, the air flow speed in a certain area down the hub 910 is very small, and it is very difficult for components in the downstream area of the hub 910 to radiate heat; while the air flow rate in an area at edges of the blades 920 is obviously too high and exceeds the air flow rate required for components in the area, resulting in that the air flows non-uniformly from an air blowing section of the whole axial fan 900. A common method to address the problem is keeping the downstream components of the hub 910 a specific distance away from the hub 910, where the distance is normally at least the diameter of the hub 910, thereby forming a pressure-balancing chamber, so that the air flow is fully mixed within a space of the pressure-balancing chamber to eliminate impacts of the hub 910 on the air flow. However, the effect is unsatisfactory. Nevertheless, in some scenarios such as for a server, a system structure is very compact, and an axial fan 900 is quite close to downstream components, so that no sufficient pressure-balancing space is available, resulting in that air flow distribution is seriously non-uniform, the air flow rate in an area down a hub 910 is obviously insufficient, and it is very difficult to radiate heat in this area, thereby causing a harmful phenomenon of overheating, affecting the working stability of devices, and shortening the service life of the devices.